In order for logic circuitry in an integrated circuit to operate, it is necessary to provide power to the logic circuits. To power logic circuits, it may be necessary to provide a power supply connection and a ground connection. Furthermore, in order to minimize power consumption when the logic circuitry is inactive, it is desired that leakage current be minimized. Therefore, it is desired to have a large on-current to off-current ratio.
Header and footer configurations are commonly used ways to provide the connections to the logic circuits, wherein a header configuration uses a switch to couple (and decouple) the logic circuits to a power supply and a footer configuration uses a switch to couple (and decouple) the logic circuits to a ground connection. Therefore, to provide power to the logic circuits, switches in both the header and the footer configurations could close, providing a current path from the power supply to the ground. For example, in a header configuration, typically PMOS (P-type Metal Oxide Semiconductor) transistors can be used as a switch to cut the power supply connection to the logic circuits, while in a footer configuration, typically NMOS (N-type Metal Oxide Semiconductor) transistors can be used to cut the ground connection to the logic circuits.
Variations in the header and footer configurations have included the use of high threshold voltage and low threshold voltage PMOS and NMOS transistors to help improve the on-current to off-current ratio. In some variations, even NMOS transistors have been used as switches in header configurations. In certain manufacturing processes, wherein it may be possible to create transistors with wide variations in geometries (and other parameters), the use of the header and footer configurations can provide both a large on-current and a small off-current.
One disadvantage of the prior art is that in many manufacturing processes wherein the ability to vary the widths of transistors is limited, it may be difficult to use standard transistors in the header and footer configurations and still provide both a large on-current and a small off-current.
A second disadvantage of the prior art is that the use of high and low threshold voltage transistors may preclude the use of a manufacturing process wherein a limited number of transistor sizes may be used. The use of transistors with limited sizes can reduce the ability to provide a satisfactory on-current to off-current ratio.